Electrical cables generally comprise one or more conductors individually coated with semiconductive and insulating polymeric materials and collectively surrounded with protective coating layers, which are also made of polymeric materials. Depending on the application, such cables may be categorized as low voltage, medium voltage, or high voltage. Typically, “low voltage” means a voltage up to 1 kV, “medium voltage” means a voltage of from 1 kV to 35 kV, and “high voltage” means a voltage greater than 35 kV.
For cables installed in critical environments such as, for example, oil refineries, oil pools, and offshore installations, the permeability of the polymeric cable coatings to humidity and, in particular, to aggressive chemicals presents a problem. These chemicals may be organic such as, for example, hydrocarbons and solvents. They also may be inorganic such as, for example, acids and bases. Penetration to the interior of the cables by the chemical elements compromises the cables' overall lifetime performance in terms of both mechanical and electrical properties.
Thus, electrical power and control cables that are exposed to chemical agents, such as in oil, gas, and petrochemical applications, must be suitable to protect insulated cores from damage caused by such chemicals.
A conventional protection against caustic elements is placement of a lead sheath in a radial internal position with respect to the outermost protective coating layer, i.e. the outer jacket. Lead provides hermetic sealing capability, and is considered relatively easy to extrude in long lengths. Cables of this type are commercially known, for example, as Solid Type PILC cables from The Okonite Company.
Alternatively, welded corrugated aluminum (or copper) sheaths are also known to afford cable protection. Aluminum sheaths are relatively light, provide hermetic sealing capability and may serve as a neutral conductor when placed over power cables. Cables of this type are commercially known, for example, as CL-X® Type cables from The Okonite Company.
In the following description, cables comprising at least a metal protective sheath shall be referred to as “metal clad cables.”
The presence of lead or aluminum sheaths adds significant weight to electrical cables. Such sheaths also can make the cables difficult to bend.
To address the limited flexibility of metal clad cables, cable installers have several options. One option is to increase the bending radius that the metal clad cable is pulled around. This approach, however, may wastefully require the use of more cable overall and would not be possible in many circumstances. Another option is to install shorter cable pulls, splicing together the shorter sections to form a desired cable length and shape. This option, however, may unnecessarily increase the installation time.
U.S. Pat. No. 7,601,915 discloses an electrical cable comprising at least one conductor, at least one metallic tape coated with at least one adhesive coating layer and at least one coating layer comprising at least one polyamide or a copolymer thereof. In a radially inner position with respect to the metal tape, a protecting coating layer made of an expanded polymeric material can be provided. In the case of a tripolar cable, the interstices between the insulated conductors are filled with a filler material that forms a continuous structure having a substantially cylindrical shape. The filler material is generally made of elastomeric mixtures or polypropylene fibres, and more preferably is made of a flame-retarding material.
Applicant has observed that the shape of the filler can affect the proper sealing of the metal tape coated with the adhesive layer. In particular, should the outer boundary of the filler material deviate substantially from circularity, the metal tape coated with the adhesive layer may fail to achieve a proper seal or may lose its seal. The loss of a tight seal in the metal tape can jeopardize the cable's ability to resist degradation from external chemical agents. In particular, Applicant has found that if fibrous materials are used as fillers, or the filler material is otherwise discontinuous, the filler may fail to attain or maintain a substantially cylindrical shape, leading to inadequate sealing of the metal tape coated with the adhesive layer.
Applicant has also observed that, while using a solid filler, such as one made of elastomeric mixtures, may be suitable for creating and maintaining a cylindrical shape, the use of a solid filler would decrease flexibility of the cable, which is also detrimental.
Other techniques for filling the interstices of a cable are known, but do not appear to provide adequate balance between the need to maintain the integrity of the metal-tape seal and to keep the cable flexible. For instance, U.S. Pat. No. 4,707,569 discloses a multi-conductor cable such as an electrical cable, a signal-transmission cable or optical fiber cable, including a core made of a plurality of insulated conductors and a sheath surrounding the core. The void space is filled with a plurality of foamed plastic string fillers between the core and the sheath and between the insulated conductors. The string is a composite string formed of a thin strip tape and a foamed plastic layer integrally provided over the surface of the tape. The tape serves as a reinforcing member and is preferably formed of a paper, a non-woven fabric or a plastic film (for example of polypropylene, polyethylene, polybutene, polyester or polyacetal). The filler strings may be used in conjunction with the conventional fillers such as slit yarns, paper tapes and the like. According to this patent, it is preferred that at least 50 vol % of the space fillers filled in the cable be occupied by the foamed plastic strings.
U.S. Pat. No. 5,113,040 discloses a flexible electrical cable including two stranded, rubber-insulated conductors. Two conductors are stranded together with two cable fillers to form a core assembly. Each cable filler includes a rubber strand having a centrally embedded bearing part, which is made up of several non-stranded, high-tensile plastic filaments. The core assembly conductors and cable fillers are first surrounded by a spun covering of a open mesh tape.
U.S. Pat. No. 3,590,141 relates, in one embodiment, to a cable comprising a layer of plastic material that can be either unfoamed or foamed polyethylene or the like and is disposed between the coaxial cables and a layer of hygroscopic material. A layer of hygroscopic material is fashioned of, for example, paper, textile cloth, blend of polymer material and siccative drying agent, or the like.
Applicant has noted that these known approaches to filling voids within a multi-polar cable do not address the problems observed with losing roundness in the outer boundary of the filler material and with having an entirely solid filler region. A design for the filler material avoiding these drawbacks and, therefore, maintaining the integrity of the metal-tape seal and the flexibility of the cable is needed. An electrical cable having such a desired filler material could reliably provide resistance to external chemical agents, provide high mechanical strength and flexibility.
For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about.” Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.